1. Field of the Invention
This invention relates to an image reading device for performing digital image signal processing, and more particularly, to an image reading device used in a film scanner or a digital film printer which reads a film original, such as a microfilm or the like, by an image sensor, such as a CCD (charge-coupled device) or the like, and performs digital image signal processing.
2. Description of the Related Art
As reader printers for microfilms, so-called analog-type reader printers have been used in which a microfilm image is irradiated by an illuminating lamp, projected light from the microfilm is enlarged into an arbitrary magnification by a projection lens, the enlarged image is subjected to slit exposure onto a photosensitive member, such as a photosensitive drum or the like, and the image on the photosensitive member is subjected to processes such as toner development, transfer and the like by an electrophotographic method to obtain a copy of the microfilm image. Recently, so-called digital-type reader printers have also been developed in which the above-described projection light is not subjected to slit exposure onto a photosensitive member, but is read by an image sensor, such as a CCD or the like, is then subjected to signal processing such as image processing and the like, and the resultant digital signal is transmitted to a printer, such as an LBP (laser-beam printer) or the like, to obtain a copy of the microfilm image. The assignee of the present application has proposed one of such printers in U.S. Pat. No. 4,700,237.
There are two kinds of microfilms, that is, a negative film and a positive film, which are used properly according to specific purposes. The assignee of the present application has proposed in U.S. Pat. Nos. 4,341,463 and 4,627,703 apparatus incorporating two kinds of image forming processes so that both negative and positive films can be reproduced (as positive copies) in an analog-type reader printer.
In such conventional analog-type reader printers having two kinds, that is, negative and positive, of image forming processes, there are the disadvantages that the circuitry and mechanism become complicated. For example, toners having different polarities are needed, two kinds of developing units must be provided, a mechanism for switching between these developing units must be provided, circuitry or a mechanism must be provided for switching polarity of the high-voltage output of a transfer charger or the like, two kinds of high-voltage power supplies must be provided due to a difference in load characteristics of high-voltage outputs, and blank exposure for providing blank must be switched. However, by setting a high-voltage output value so that an optimum .gamma. characteristic (exposure amount vs density characteristic) can be obtained for both negative and positive films, reproduced images having excellent gradation can be obtained for both negative and positive films.
To the contrary, in a digital-type reader printer, since, basically, a positive copy (a positive reproduced image) can be obtained from either a negative film or a positive film by only outputting an inverted digital signal, complicated circuitry and mechanism are not specifically needed as in the case of an analog-type reader printer.
However, it is impossible to obtain a reproduced image having excellent gradation by only inverting a digital signal as described above. One of the main reasons is that the .gamma. characteristic of a film is not linear, but is nonlinear, and curves near points A and B in the .gamma. characteristic curve of a film shown in FIG. 16 are not symmetrical. Accordingly, when a positive original is photographed onto a film, the portion A corresponds to a line-drawing portion, and the portion B corresponds to a background portion. To the contrary, when a negative original is photographed onto a film, the portion B corresponds to a line-drawing portion, and the portion A corresponds to a background portion. Hence, there is the problem that gradation becomes different between a negative film and a positive film.
In general, in the case of a positive film, since dust, stain, scratches and the like on the film have densities close to those of line-drawing portions, they become considerably emphasized if sharpness of the image is too strong, and so a dirty image is obtained. To the contrary, in the case of a negative film, since dust and the like on the film have densities close to those of background portions, they do not become emphasized even if sharpness of the image is increased.
As can be understood from the foregoing description, the degree of sharpness must be changed between a negative film and a positive film. In a conventional digital-type reader printer, however, an output signal from an image sensor, an input signal from a reception device, or the like is inverted in accordance with the kinds of negative/positive films. Hence, although complicated circuitry and a complicated mechanism as required in the case of an analog-type reader printer are not needed, there is the disadvantage that sharpness is so strong for a positive film that dust, stain, scratches and the like on the film are emphasized, and so a dirty image is obtained since edge emphasis processing is not changed in accordance with the kinds of negative/positive films.